Applying a titanium-containing substance onto a surface of any of various materials, for example any of various building materials such as glass, a ceramic or a tile, to form a photocatalytic coating film comprising titanium oxide (titania), and thus realizing any of various functions such as an anti-fouling function, an antimicrobial function, a gas decomposing function or a harmful organic matter decomposing function realized through the photocatalytic ability of the coating film, is something that has been carried out from hitherto. Regarding the method of forming such a titanium oxide coating film, for example a method is known in which a dispersion containing fine particles of a titanium oxide or a solution of a titanium compound is applied onto the substrate surface, and then after the application, drying or if necessary low-temperature baking is carried out.
In particular, it is known that as the titanium oxide used for realizing the photocatalytic ability, not only titanium dioxide such as anatase-type or rutile-type titanium oxide, but also a titanium oxide having peroxy groups, i.e. titanium peroxide, can be used. With regard to this titanium peroxide, it is also known that only the anatase type has catalytic ability (Japanese Patent Application Laid-open No. 9-124865).
Due to such anatase-type titanium peroxide having photocatalytic ability as mentioned above, the above-mentioned patent document also describes using this anatase-type titanium peroxide as a photocatalytic film by forming a coating film thereof on a substrate surface of any of various structures.
Moreover, regarding the amorphous type, there is no photocatalytic ability, but the bonding ability is excellent, and hence use as a photocatalyst particle binder when forming a photocatalytic coating film has been proposed (Japanese Patent Application Laid-open No. 9-262481).
However, a coating film formed from anatase-type titanium peroxide has photocatalytic ability as described above, and yet the electrical conductivity is insufficient, and hence such a coating film has not been adequate for use as an electromagnetic wave shield, an antistatic material or the like. A coating film formation technique for improving the electrical conductivity has thus already been proposed (Japanese Patent Application Laid-open No. 11-315592), but this coating film formation technique has not been adequate since the formation process is complex.
To resolve such problems, the present inventors have developed a titanium peroxide-containing aqueous liquid that enables a coating film having photocatalytic ability, and electrical conductivity useful for electromagnetic wave shielding, prevention of static and so on to be formed more easily, and have already filed patent applications (Japanese Patent Application No. 2001-4506, Japanese Patent Application No. 2001-250468). However, although this titanium peroxide-containing aqueous liquid has high functionality and is easy to use, the hardness has not been sufficient for forming a serviceable coating film having photocatalytic ability on glass, resin sheets, metal plates and so on. The present inventors thus focussed on this point, and also developed art for a coating film forming liquid capable of forming a high-hardness photocatalytic coating film, and have already filed a patent application (Japanese Patent Application No. 2002-11493).
[Problems to be Solved]
However, in the case of forming a coating film on the surface of glass, metal or the like using a conventional titanium peroxide-containing aqueous liquid as described above or a titanium oxide-containing liquid in which titanium oxide is dispersed in a liquid containing a substance having an alkyl silicate structure, it has been observed that under environments in which the photocatalytic ability is not readily exhibited, for example in bathrooms or washrooms or inside automobiles, fogging may arise on mirrors or on glass surfaces inside automobiles, and hence the hydrophilicity and anti-fogging ability have not been adequate for mirrors, glass and so on.
[Object of the Invention]
It is an object of the present invention to provide art enabling formation, on the surface of any of various substrates, of a coating film that has photocatalytic ability and moreover is super-hydrophilic in a state in which the photocatalyst is not excited. Specifically, it is an object of the present invention to provide an aqueous liquid capable of forming, on the surface of a substrate such as glass, a ceramic or metal, a coating film that is super-hydrophilic in a state in which the photocatalyst is not excited, a structure having this coating film, and a method of manufacturing the structure having the coating film. Moreover, it is an object of the present invention to provide art enabling formation of a coating film that preferably not only has super-hydrophilicity but also has water-retaining ability, wear resistance (high hardness) and transparency.